The voltage generated by a single battery cell is determined by active materials constituting the cell and is typically a low value on the order of no more than several volts. For this reason, multiple battery cells are typically used to form a battery pack.
In addition, for technical reasons such as the difficulty of manufacturing high-ampacity battery cells, safety and economic reasons when there is a need for high-ampacity battery packs, multiple cell strings are connected in parallel in the battery pack.
Japanese Patent Application Laid-Open No. 2012-234700 discloses an example of the prior art where multiple cell strings or the like are connected in parallel in battery packs.
In battery packs including a configuration in which multiple cell strings are connected in parallel, there is a need to ensure the potentials of the respective cell strings exactly the same. Otherwise, there will be risks such as ignition, due to current flowing in without any limitation from the cell strings or the like at potentials to the cell strings at low potentials. However, in order for the potentials of relatively high-capacity cell strings or the like that have an ampacity of several Ah or more to be made coincident, specifically, there is a need for work to connect the cell strings or the like in parallel with resistors interposed therebetween and leaving the cell strings or the like for a long period of time, thereby increasing the manufacturing cost of the battery packs.
In addition, each of the cell strings has to have substantially the same electrical characteristics. Originally, unless the charge voltage and discharge voltage of the cell strings or the like are coincident, it is obviously impossible to connect the cell strings or the like in parallel, and cell strings or the like have necessarily to be used which have the same type and the same number of battery cells connected in series.
Moreover, if some cell strings having a high internal resistance are connected in parallel with other cell strings that have a low internal resistance in a battery pack, an imbalance will be caused with the result that more charge and discharge currents flow through the strings that have a low internal resistance, and the ampacity of the battery pack as a whole has to be thus set to a value that is significantly lower than the total of the ampacities of the cell strings constituting the battery pack.
For these reasons, it is not only impossible to use different types of cell strings in mixture, but also it is not preferable to use new strings together with cell strings as used items. This is because the performance of the cell strings is degraded by over time. Preferably, battery cells that are used for the cell strings have to be obtained by selecting battery cells that have close electrical characteristics from the same manufacturing lot.
As just described, the increased manufacturing cost of battery packs, the difficulty with maintenance when some cell strings have broken down, the difficulty with the use of used batteries are problems in the manufacture of battery packs that have multiple cell strings or the like connected in parallel. In addition, when new cell strings are to be added to multiple cell strings or the multiple cell strings are to be replaced by new cell strings, it is anticipated that simply connecting the cell strings in parallel to each other may result in problems such as abnormal overheating due to rapid current flowing as a function of the magnitudes of the voltages between the cell strings.
Therefore, a main object of the present invention is to provide a battery pack which can increase the productivity and operability of the battery pack in which multiple secondary batteries (storage battery) exhibiting different characteristics from each other are housed.